In order to remain competitive, refiners have continuously sought to improve the quality of middle distillate products while simultaneously reducing processing costs. Refiners have recently sought to maximize existing equipment to achieve desired upgrades rather than build new equipment, in order to control costs. Such maximization is a continual challenge to refiners, since refining stocks have become heavier and poorer in quality. Upgrading capacity has been further strained by more stringent mandates on emissions.
FCC cycle oil is a feed commonly used for the production of middle distillates and automotive diesel fuel. FCC cycle oil is a broad cut boiling between about 300.degree. F. and 900.degree. F. In addition to paraffins and cyclo paraffins, it contains both two and three ring aromatic structures and thiophenes. The thiophenes are generally multiple ring structures, such as benzothiophene, dibenzothiophene, substituted benzothiophenes and substituted dibenzothiophenes.
Combined processing of both heavy and light portions of FCC cycle oil negatively affects hydroprocessing operations, such as catalytic hydrodesulfurization (CHD) and hydrocracking (HDC), including pressure requirements, flow rates, temperatures, product quality, and product yields. Operating conditions are generally dictated by the larger structures and are excessively severe for the lighter portion, which contains the smaller molecular structures. Catalysts tend to age relatively quickly when employed under excessively severe operating conditions, also.
Zeolites have not been employed frequently as the support in commercial catalysts for mild hydroprocessing for heteratom removal and bond saturation (such as CHD), either on their own or combined with an amorphous matrix such as alumina because they tend to have a greater activity than alumina or other commonly used supports. With activity increase there is a concommitant increase in boiling range conversion and reduction in distillate yield. In addition, acidic zeolites are subject to coke formation and rapid aging under mild hydroprocessing conditions with feeds boiling above about 550.degree. F.
Zeolites, especially zeolites X and Y, have long been used in more severe hydroprocessing operations such as hydrocracking, where their relatively greater activity is an asset. Under hydrocracking conditions they have excellent resistance to aging, particularly the more highly siliceous forms of zeolite Y, such as "ultra-stable" or USY.
There are regulations throughout the world on the permissible quantity of sulfur in distillate products. The Environmental Protection Agency (EPA) and state environmental agencies, such as the California Air Resources Board (CARB) have established maximum standards of 0.05 wt % sulfur, for example. These standards went into effect in 1994.
Various means have been proposed to upgrade feeds of high aromatic content. U.S. Pat. No. 4,789,457 discloses the recycling of full range cycle oils or cycle oil fractions to a catalytic cracking unit, where such feeds are subjected to low pressure hydrocracking in order to maximize the production of high octane gasoline.
In order to avoid aging, conversion should be limited when operating with full range light cycle oil, and lower boiling fractions are preferred. U.S. Pat. No. 5,011,593 discloses the treating of full range cycle oils (boiling in the range of 385.degree.-750.degree. F.) or fractions thereof by catalytic hydrodesulfurization employing zeolite beta and a hydrogenation component.
U.S. Pat. No. 3,957,625 discloses that sulfur impurities tend to concentrate in the heavier portion of a product fraction. It proposes a method of removing the sulfur from catalytically cracked gasoline by hydrodesulfurization of the heavy portion of the gasoline. The octane contribution of the olefins found in the lighter fraction is therefore retained. The light and heavy gasoline fractions are then recombined following separate treating.
U.S. Pat. No. 4,990,242 is concerned with enhanced removal of sulfur from fuels. This patent discloses the fractionation of a feedstock and the separate removal of sulfur from the lighter fraction. On splitting the full range cycle oil at 570.degree.-575.degree. F. and separately hydrotreating the two fractions, the light portion attains a sulfur level below the 0.05 wt. % S standard. When combined with the separately hydrotreated heavy fraction, however, the standard is not met.